Membrane electrode assembly (MEA), method for its manufacturing and a method for preparing a membrane to be assembled in a MEA

ABSTRACT

A method for preparing a membrane to be assembled in a membrane, electrode assembly includes the step of swelling an ion-conducting membrane in a liquid containing at least one solvent or to an atmosphere containing the vapor phase of at least one solvent by controlling the content of the solvent in the ion-conducting membrane. A method for manufacturing a membrane electrode assembly using an ion conducting membrane includes the steps of: providing an ion-conducting membrane in a pre-swollen state; coating the ion-conducting membrane on both sides with an electrode layer to form a sandwich; and hot-pressing the sandwich to form an ion-conducting bonding of the layers of the sandwich. Furthermore, a membrane electrode assembly is disclosed including a hot pressed sandwich having an electrode layer, a ion-conducting membrane and again an electrode layer, thereby using the ion-conducting membrane in its pre-swollen status prior to the hot-pressing.

FIELD OF THE INVENTION

The invention relates to a method for manufacturing a membrane electrodeassembly and to a method for preparing a membrane to be assembled in anmembrane electrode assembly. Further, the invention relates to amembrane electrode assembly.

BACKGROUND OF THE INVENTION

Rapid and simple methods for creating Membrane Electrode Assemblies(MEAS) having optimized interfaces are desired. As discussed by Huslageet al. [J. Huslage, T. Rager, B. Schnyder, and A. Tsukada“Radiation-grafted membrane/electrode assemblies with improvedinterface” Electrochim. Acta 48 (2002) 247-254], the preparation of suchoptimized interfaces with radiation-grafted crosslinked membranes hasbeen a long-standing problem in the field. In particular, they notedpoor electrochemical interface formation is characterized by unstablefuel cell performance, lengthy run-times until steady-state performanceis achieved, and poor adhesion between membrane and electrodes evenafter extended fuel cell testing. Therefore, it is clear that theoptimization of the electrochemical interface of MEAs is quite importantin obtaining high performance in the fuel cell application. For example,the optimization of this interface allows one to obtain better cellpolarization performance and higher power densities with the same fuelcell components, e.g. membrane and electrodes etc.

As discussed by Huslage et al., it is especially difficult to obtain anoptimal electrochemical interface in MEAs containing radiation-graftedcrosslinked membranes having low graft levels. This interfacial problemis quite important to solve because both Huslage et al. and later Kuhnet al. (H. Kuhn, L. Gubler, T. J. Schmidt, G. G. Schmidt, H.-P. Brack,K. Simbek, T. Rager, and F. Geiger, “MEA Based on Radiation-GraftedPSI-Membrane: Durability and Degradation Mechanisms”, Proceedings of the2^(nd) European PEFC Forum, 2-6 Jul. 2003, ISBN 3-905592-13-4, pages69-77.) demonstrated that such membranes can operate in fuel cells understeady state conditions for several thousands of hours with noobservable loss in the output of the fuel cell or degradation inmembrane or cell properties or performance.

The typical preparation of such radiation-grafted membranes is describedin the above-cited publication by Huslage et al. or in, for example, theEuropean patent EP 0 667 983 B1. Often, they are prepared from 25 μmthick films of FEP base polymer and using monomer solutions containingabout 10 vol % of the crosslinker, DVB. These membranes typically havegraft levels of about 18 to 20 mass percent, and their spectroscopicproperties and the isomer ratios of the graft component have beenreported by Brack et al. (H.-P. Brack, D. Fischer, M. Slaski, G. Peter,and Gunther G. Scherer, “Crosslinked Radiation-Grafted Membranes,Proceedings of the 2^(nd) European PEFC Forum, 2-6 Jul. 2003, ISBN3-905592-13-4, pages 127-136).

Huslage et al. reported that mechanical properties of the membranes wereimproved by limiting the graft level to such rather low values, and thatthe corresponding losses in conductivity at low graft levels could becompensated by an improvement in the membrane/electrode interface.

According to the publication by Huslage et al., “MEAs with an improvedinterface between the membrane and commercially available gas diffusionelectrodes were obtained by Nafion®-coating of the membrane andhot-pressing.”

Unfortunately, these progress could not be successfully reproduced.Working according to the teaching of Huslage et al. by preparingoptimized MEAs from radiation-grafted membranes by Nafion®-coating ofthe membrane and hot-pressing delivered the quite surprisingly resultthat alone these two steps of (1) Nafion-coating and (2) hot-pressing donot make it possible to obtain MEAs having favorable electrochemicalproperties such as low ohmic or charge-transfer resistances or havingfavorable performance properties in fuel cells.

In the prior art studies, documents published by Supramanium Srinivasanwere found emphasizing the major breakthrough obtained in making a10-fold reduction in platinum loading from about 4 mg cm⁻² to 0.4 mgcm⁻² or less. For example, S. Srinivasan and P. Costamanga review thisbreakthrough in “Quantum jumps in the PEMFC science and technology fromthe 1960s to the year 2000 Part I. Fundamental scientific aspects”, J.Power Sources, 102 (2001) 242-252. He attributes this breakthrough to(i) an increase in the BET surface energy of the electrocatalysts, and(ii) an extension of the 3-dimensional electrochemically active zone inthe electrode by the impregnation with the proton conductor Nafion®membrane. In this same publication, S. Srinivasan et al. state that theelectrodes are hot-pressed to the membrane “under the desired conditionsof temperature (130-140° C.), pressure (2000 psi) and time (about 1min)”. Unfortunately, S. Srinivasan et al. did not state howeveranything about a pretreatment or pre-conditioning of the membrane priorto the hot-pressing.

In addition, their teaching that hot-pressing should be done close tothe glass-transition temperature is ambiguous. It is not clear if theymean the glass-transition temperature of the form of the membrane duringhot-pressing or any other type of a pretreated membrane. Also, the term“close to” is not specified and could mean one degree or many degreesCelsius. Nor is it indicated whether there is difference or preferencebetween hot-pressing below, at, or above the glass transition.

SUMMARY OF THE INVENTION

It is therefore the aim of the invention to provide a method for themanufacturing of a membrane electrode assembly and a method forpreparing a membrane for use thereof and a membrane electrode assemblyproviding improvements with respect to bonding, lifetime and performanceproperties.

Spoken in general terms, the inventive concept comprises to control thewater-content of the membrane during hot pressing because this parameterturned out to be critical to give an optimized MEA interface andfavorable electrochemical properties and fuel cell performance. Thisinvention and its various embodiments and reduction to practice will bedescribed later and can be studied from the appending claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts typical cell polarization curves of fuel cells containingthe various MEAs described in the working and comparative examples.

FIG. 2 depicts typical results of electrochemical impedancespectroscopic measurements of fuel cells containing the various MEAsdescribed in the working and comparative examples.

DETAILED DESCRIPTION OF THE INVENTION

Therefore, an ion-conducting membrane, in a swollen state characterizedby a relatively high surface energy or low contact angle with water forthat material, is bonded together with two electrodes under conditionsof elevated pressure and/or temperature. The state of swelling of theion-conducting membrane is conveniently varied by exposing the membraneto one or more liquid solvents or to atmospheres containing the vaporphase of one or more solvents. The method of membrane swelling is notspecifically limited. Non-limiting examples such solvents include water,ethylene glycol, propylamine, propanol, propionic acid, andpropionaldehyde, acetone, acetonitrile, N-butyl glycolate,N,N′-di-n-butylacetamide, diethoxyethane, diethyl carbonate,1,3-dioxolane, dimethylacetamide, N,N′-dimethyl butyramide, dimethylcarbonate, N,N′-dimethyl decanamide, dimethoxyethane, dimethyl ethanesulfonamide, N,N′-dimethyl formamide, N,N′-dimethyl propyleneurea,dimethyl sulfoxide, dimethyl sulfite, 2,5-dimethoxytetrahydrofuran,ethyl acetate, 2-ethoxyethyl acetate, ethylene carbonate(1,3-dioxolan-2-one), ethyl glycolate, CH₃OC₄F₉,CF₃CF₂CF₂OCF(CF₃)CF₂OCHFCF₃, γ-butyrolactone, methyl acetate,2-(2-(2-methoxyethoxy)ethoxy)-1,3-dioxolane, methanol, methyl formate,methyl glycolate, methyl tert-butyl ether, N-butyl amine, N-methylformamide, N-methyl-2-pyrrolidone, propylene carbonate, poly(ethyleneglycol), 4-(1-propenyloxymethyl)-1,3-dioxolan-2-one, sulfolane,tetrabutylammonium, triethylphosphate, tetrahydrofuran, and 3-methylsydnone. In one embodiment, an ion-exchange membrane is treated indeionized water for about 4 or 5 h at a temperature of about 80° C.

The type of membrane used in this invention is not specifically limited.Any organic or inorganic or organic/inorganic or composite membranecapable of transporting protons at the operating temperatures ofinterest is suitable. The organic component may be polymeric in nature.The composition and chemical structure of the membrane is also notlimited. The membrane will generally be selected according to thedesired properties for the membrane, for example, conductivity,dimensional stability, gas separation, methanol impermeability ormechanical properties. Some non-limiting examples of ionomeric membranesinclude copolymers of fluorinated monomers like tetrafluoroethylene andion-conducting or acidic comonomers or their precursors. Non-limitingexamples of such polymers are the perfluorosulfonic acid (PFSA) polymerNafion® membranes by DuPont or related materials from Asahi Glass(Flemion membrane), Asahi Chemical, or Dow. Such ionomeric membranematerials may also be used in the form of composites, such as in thecase of the composite micro-reinforced membrane found in the PRIMEA® MEAfrom Gore. The composition, shape and form of the membrane are notspecifically limited. Such perfluorinated membranes are often cleanedand put fully into the acid form prior to MEA assembly by treating themfor short periods of time with strong acid solutions. In one embodimentthey are treated with about a 35 weight % solution of nitric acid forone hour prior to swelling. After acid treatment, the acid is removedfrom the membranes typically by rinsing the membrane with water untilthe rinse water is neutral.

The electrochemical performance of the MEA and fuel cell in some casesmay be enhanced by impregnation of the membrane with another ionicallyconducting polymeric phase in order to extend the volume of the3-dimensional electrochemically active zone or to improve the surface orbonding properties of the membrane in the MEA preparation. Thisimpregnation can be carried out by means known in the art such asspraying or dipping the membrane with a solution of an ionicallyconducting polymer such as the Nafion® polymer or other perfluorinated,partially fluorinated, or non-fluorinated ionomers. After theimpregnation process is complete, the membrane may be dried at elevatedtemperatures, typically 100 to 140° C. in the case of Nafion® polymer,in order to remove residual solvent and to transform the ionomer intothe form of an insoluble solid.

In one embodiment, the membrane is a radiation-grafted membrane. Thecomposition and structure of the radiation-grafted-membrane is notspecifically limited. Generally, the graft level shall be in the rangeof 10 to 40 mol % as compared to the original amount of membranematerial. For example, the radiation-grafted membrane may be preparedfrom a variety of base polymer films including fluoropolymers, such asPTFE and its copolymers including FEP, ETFE, or PVDF, or polyolefinssuch as polyethylene, polypropylene and their copolymers. The type ofradiation used to prepare the membrane is not specifically limited andincludes electromagnetic radiation like UV or X-rays or particleradiation such as electron beam. In some cases a vacuum or inertirradiation atmosphere will be selected in order to minimize degradationof the base polymer. Films can be grafted simultaneously along with theirradiation process, or the grafting can be done in a post-irradiationstep.

If the grafting is done in a subsequent step, the irradiated substratemay be stored at reduced temperature and/or under inert atmosphere ifthe reactive sites are unstable. The physical form of the monomer forgrafting may be as a gas or a liquid, and the monomer may be either pureor diluted with a solvent or inert material and/or as a mixture with oneor more additional monomers. Any radically active monomer may be usedincluding vinyl, styrenic or acrylic monomers. Monomers can be selectedaccording to the properties that are desired for the membranes. Forexample, if it is desired that the membrane conduct ions, monomershaving acidic, basic, salt or amphoteric functionality or theirprecursors may be selected. Non-limiting examples of monomers havingacidic, basic, salt, or amphoteric functionality include vinylphosphonic acid, vinyl sulfonic acid,3-[(2-acrylamido-2-methylpropyl)dimethylammonio]propanesulfonate, sodiumstyrene sulfonate, N-vinyl-2-pyrrolidone, 4-vinyl pyridine. Monomersthat can be used as precursors for the introduction of acidic, basic,and amphoteric groups may also be used. Non-limiting examples includestyrenic monomers such as styrene, α,α,β-trifluorostyrene,α-fluorostyrene, and vinylbenzyl chloride and their derivatives.Crosslinking monomers known in the art, such as divinyl benzene orbis(vinyl phenyl)ethane, can be used to modify the swelling, gas orliquid crossover properties, or stability and durability of suchmembranes. The content of such crosslinking monomers in a graftingsolution shall be in the range of some percent and shall not exceed 25%,preferably not exceed 20%. The solution may be added by styrene to 100%.The grafted polystyrenic chains can later be derivitized using methodsknown in the art to yield acidic, basic, or amphoteric functionalitiesin the membrane. For example, anion-exchange groups can be introduced bymeans of subsequent amination followed by ion exchange with aqueoushydroxide, and cation exchange groups can be introduced by treating withstrong acids such as chlorosulfonic or sulfuric acids or sulfur trioxidedissolved in halogenated solvents. The fully acid forms of suchmembranes are typically obtained by treating them first with basicsolution, followed by regeneration in acid, and finally rinsing withwater. In one embodiment, a freshly sulfonated membrane is treated firstwith 0.1 M NaOH for at least about 12 h and then for about 5 h in 2 MH₂SO4.

The electrode shapes, forms, structures and compositions are notspecifically limited. Generally they will be electronically conductingand additionally nay have catalysts present on or in them. Often theywill have the capability to allow the transport or diffusion of gaseousor liquid reagents through at least some regions of their structure.Non-limiting examples of electrode materials include carbon cloth,carbon paper, or carbon felt. For the case of fuel cell electrodes,additional catalysts may used in order to catalyze electrochemicaloxidation or reduction reactions. Non-limiting examples of catalystsinclude platinum on carbon, platinum black or platinum alloys with othermetals such as ruthenium or metal oxides, and Raney nickel together withcarbon blacks. Some of these alloys may be applied to advantage inminimizing the poisoning of electrode surfaces with CO and other speciesand in catalyzing the oxidation of methanol, other alcoholic species, oreven hydrocarbons. The electrochemical performance of the electrode, MEAand fuel cell can be significantly enhanced by impregnation ofelectrodes with an tonically conducting polymeric phase in order toextend the volume of the 3-dimensional electrochemically active zone.This impregnation can be carried out by means known in the art such asspraying, dipping, or wetting the surface of the electrode with asolution of an ionically conducting polymer such as the Nafion® polymeror other perfluorinated, partially fluorinated, or non-fluorinatedionomers. After the impregnation process is complete, the electrode maybe dried at elevated temperatures, typically 100 to 140° C. in the caseof Nafion® polymer, in order to remove residual solvent and to transformthe ionomer into the form of an insoluble solid.

The preparation of a membrane electrode assembly according to theinvention can be described as follows: An ion-conducting membrane ispre-swollen in a solvent such as water. Care is taken that the membraneremains in a swollen, plasticised state characterized by an increasedsurface energy and more hydrophilic surface during the MEA assemblyprocess. The effects of swelling on the mechanical and surfaceproperties of the membrane can be readily varied by the choice ofsolvent.

For example, polar and hydrogen-bonding solvents will better swell andthus more greatly influence the mechanical and surface properties ofmembranes containing polar and hydrogen-bonding functional groups likeacidic, basic, or amphoteric ones. The extent of swelling of themembrane can be readily controlled by methods known to those art,including the volatility of the chosen solvent, the exposure time to thesolvent and its concentration, the means of solvent exposure, theatmosphere to which the membrane is exposed to after swelling, and theduration of this exposure prior to hot-pressing of the MEA. In oneembodiment, a coating of catalyst may be applied to the membrane priorto MEA assembly by means of spraying, dipping, sputtering or othermethods known in the art.

Typically the swollen form of the membrane is then placed in intimatecontact with two electrodes, one on each side. The MEA sandwich is thenhot-pressed for a period of time at elevated pressure and temperature.The exact hot-pressing conditions are not specifically limited, and theoptimum conditions will depend somewhat on the membrane and electrodeproperties such as hardness, surface energy, and mechanical and chemicalstability. Various combinations of the conditions of temperature,pressure and time may be made. For example, the use of highertemperatures will generally make it possible to use somewhat shorterhot-pressing times or lower pressures. In another case, the use ofhigher pressures will make it possible to use somewhat lowertemperatures and shorter hot-pressing times. In yet another case, theuse of longer pressing times will make it possible to use lowertemperatures and pressures. Typically hot-pressing temperatures ofbetween about 60 and about 150° C. may be used. In one embodiment thetemperature is about 110° C. In general, the use of too hightemperatures in undesired because it may lead to membrane drying or evendecomposition, especially in the presence of oxygen. Too low atemperature may lead to poor MEA bonding. Typically hot-pressingpressures of between about 2 and about 30 MPa may be applied. In oneembodiment the applied pressure is about 5 to about 18 MPa. Again, toohigh pressure may lead to degradation, and too low a pressure may leadto poor bonding. The duration of the hot-pressing treatment may varybetween about 15 sec and about 10 minutes. In one embodiment, theduration is about 3 min.

The applications of these NEAs are not specifically limited. These MEASmay find application in a variety of electrochemical processes, cells,and devices, for example, in fuel cells, electrolysis cells, andbatteries. Such electrochemical cells may be used individually or inassemblies of several cells connected in series or parallel. The fuelcells may be powered using a variety of fuels in either gaseous orliquid form, such as hydrogen, methanol, or reformate either in a pureform or in mixtures with other components. The fuel cells may operateusing a variety of oxidants in either gaseous or liquid form, such asoxygen or air either in a pure form or in mixtures with othercomponents.

This invention was first reduced to practice in a collaboration of theFuel Cells Group and Materials II Group of the Laboratory forElectrochemistry (both at Paul Scherrer Institut, Villigen,Switzerland). Radiation-grafted membranes were prepared. They were madefrom 25 μm thick FEP films (FEP 100 A) purchased from DuPont,Circleville, Ohio, USA. It should be pointed out that the depth of themembrane shall be in the range of 5 to 250 μm, preferably 20 to 200 μm.The membranes were prepared according to the method described by Huslageet al. and they are referred to here as FEP-25 membranes. An irradiationdose of 3 kGy and reaction times of 3.5 h at 60° C. were used. Thegrafting solutions containing 10 vol % of the crosslinker, DVB, relativeto styrene. The acid form of the membranes were swollen in a deionizedwater bath at 80° C. for about five hours. The graft levels of theresulting membranes were between 18 to 20 mass %, and the ion-exchangecapacities were between 1.25 and 1.35 mEq/g.

For comparison purposes the membrane Nafion® N-112 membrane waspurchased from the DuPont company. This membrane material was firsttreated for one hour at 90° C. in a 1:1 (vol:vol) solution ofconcentrated HNO₃ (65%) in deionized water. Next the Nafion® membranewas repeatedly treated by swelling over several hours at about 95° C. inseveral baths of deionized water until the bath water remains neutral.This membrane has an ion-exchange capacity of about 0.9 mEq/g and awater swelling of about 20 mass %.

All swollen membrane materials were stored in deionized water until theywere processed in the fabrication of MEAs by hot-pressing.

The electrodes in these working examples were carbon cloth basedelectrodes of the type ELAT from E-TEK with a Pt loading of 0.6 mg cm⁻².The electrodes were Nafion-coated by spraying them with a 0.5 mass %solution of Nafion followed by drying under vacuum for about 2 h at 130°C. The amount of Nafion applied was about 0.6-0.7 mg/cm².

The membranes that were Nafion impregnated were dried in a vacuum ovenat 120° C. for 1 hour. The membranes were then immersed in 0.5 wt %Nafion® ionomer solution. After 1 hour, the membranes were taken out,the solution on the surface was gently shaken off, and the samples wereleft to dry in the fume cupboard at room temperature for about 1 hour.Subsequently, the Nafion® ionomer coating was insolubilized by curingthe membranes in the vacuum oven at 120° C. for 2 hours. The membranewhich was then to be bonded to the electrodes in wet state wasre-swollen by immersing the sample in water at room temperature.

Another process for impregnating the radiation grafted membranes withsoluble Nafion® ionomer leaves the membrane less brittle after thetreatment. In this milder process, the membranes are dried in a vacuumoven at 60° C. for at least one hour, preferably a few hours. Themembranes are then immersed in 0.5 wt % Nafion® ionomer solutionovernight. The membranes are taken out and the excess solution is gentlyshaken off. The samples are left to dry in the fume cupboard for about 2hours at room temperature. The membranes are subsequently cured in thevacuum oven at 60° C. for 2 hours.

Four types of NEAS based on FEP25 radiation-grafted membranes wereevaluated for performance and compared against a standard MEA comprisingNafion® 112. Two samples were used as prepared (not impregnated withNafion® ionomer). They were stored in water after membrane preparation.

Working Example 1

In experiment V150, an FEP-25 membrane having a graft level of 19.9% wasused. This membrane was not impregnated with Nafion® ionomer prior tohot-pressing. The membrane was hot-pressed in the swollen state (FEP-hp(wet)) using ordinary hot-pressing conditions (120° C./18 MPa/3 min).The MEA was checked for leaks and then tested in a hydrogen/oxygen fuelcell test stand as described below.

Comparative Example 1

In experiment V203, a MEA was prepared from an FEP-25 membrane having agraft level of 19.5%. This membrane was also not impregnated withNafion® ionomer prior to hot-pressing. This membrane was dried in theoven at 60° C. for 1 hour (no vacuum) prior to hotpressing (FEP-hp). Allof the other MEA preparation conditions were the same as given above.

Working Example 2

In experiment V213, an FEP-25 membrane having a graft level of 19.0% wasused. The membrane was impregnated with Nafion® ionomer as described inthe previous section. This sample was hot-pressed in the wet state,having been re-swollen at room temperature in water overnight (FEP-ni-hp(wet)). The membrane was removed directly from water and the surfacewater was removed by blotting with a tissue. The membrane was thenplaced in contact with two electrodes. The MEA sandwich was then rapidlyplaced in a hot press, and laminated under milder conditions (110° C./5MPa/3 min) because it was found that impregnated membranes are prone tocracking when using the standard hot-pressing conditions. The milderbonding conditions did not leave to an observably poorer quality oflamination.

Comparative Example 2

In experiment V211, a MEA was prepared from an FEP-25 membrane having agraft level of 17.9%. The membrane was impregnated with Nafion® ionomerand the MEA prepared according to the method of working example 2(V213), except that the membrane in this comparative example was bondedin the dry state, after curing of the Nafion coating at 120° C.,(FEP-ni-hp), after the sample had been removed from the oven fromcuring, as described above.

Comparative Example 3

In experiment V208, an MEA was fabricated from a Nafion® 112 membrane inthe wet state (N112-hp (wet)).

An overview of the MEAs fabricated is shown in Table 1.

TABLE 1 Membranes fabricated to MEAs, using ETEK electrodes with 0.6mg_(Pt) cm⁻². Degree of IEC grafting [mmol Membrane Nafion ® HotpressingMEA [%] g⁻¹] state impregnation conditions COMP EX 3 — 0.95 wet — 120°C./18 N112- MPa/3 min hp(wet) COMP Ex 1 19.5 1.44 dry No 120° C./18FEP-hp MPa/3 min WE 1 FEP- 19.9 wet No 120° C./18 hp(wet) MPa/3 min COMPEX 2 17.9 1.35 dry Yes 110° C./5 FEP-ni- MPa/3 min hp WE 2 FEP-ni- 19.01.33 wet yes 110° C./5 hp(wet) MPa/3 min

These MEAs were tested in the test stands of the fuel cells group atPSI. Fuel cell testing was carried out in single cells with 30 cm²active area. The cells were operated with pure H₂ and O₂, using a gasflow rate of 1.5 times the amount required by the cell current. Celltemperature was 80° C., pressure of the reactant gases 1 bar_(a) at theoutlet. The hydrogen was humidified by bubbling through a water at atemperature of 80° C., the oxygen was fed to the cell withouthumidification. During startup, the cells were operated at a constantinternal resistance of 0.05 ω, until performance of around 5 W wasattained. Then, the operating mode was switched to constant current of14.6 A (500 mA cm⁻²).

In situ Characterization of MEAs Once the MEAs had been conditionedafter start up and attained stable performance, the properties of theMEAs were characterized in situ by polarization experiments andelectrochemical impedance spectroscopy. Typical cell polarization curvesfor the various MEAs are shown in FIG. 1. For the polarizationexperiment, the cell current density was varied in steps from opencircuit voltage to the maximum current density. Equilibration time ateach point was 20 S. Electrochemical impedance spectroscopy was carriedout at the constant cell current density of 500 mA cm⁻² using animpedance kit from Zahner Elektrik (Kronach, Germany). Perturbationfrequency was typically varied from 100 mHz to 25 kHz. Typical resultsfor the characterization of the MEAS described in the working andcomparative examples are shown in FIG. 2. The ohmic and charge transferresistances measured in-situ by means of this electrochemical impedencespectroscopy method are summarized in Table 2.

Summary of Results: FIG. 1 shows typical cell polarization curves offuel cells containing the MEAs of the working and comparative examples.The polarization properties of the cells containing MEAs based on theradiation-grafted membranes are significantly improved if the MEA ishot-pressed while the membrane is in a swollen state. This improvementis significant whether or not the membranes are impregnated with Nafion®ionomer prior to hot pressing (comparison of working example 2 andcomparative example 2, and working example 1 versus comparative example1, respectively).

TABLE 1 Summary of MEA resistances of working and comparative examplesmeasured in-situ by means of electrochemical impedence spectroscopyExperiment R_(ohm) R_(CT) Example Number mOhm cm⁻² mOhm cm⁻² WorkingExample 1 V150 132 222 Comparative V203 132 358 Example 1 WorkingExample 2 V213 124 229 Comparative V211 129 297 Example 2 ComparativeV208 101 195 Example 3

FIG. 1 Typical cell polarization curves of fuel cells containing thevarious MEAs described in the working and comparative examples.Measurements were made after the MEAs had been conditioned subsequent tostart up and attaining stable performance, typically about 100-150hours. V150 (FEP25-hp (wet)) was actually measured after about 2,350hours, but it had not degraded in performance versus the earliermeasurements with this MEA.

FIG. 2 Typical results of electrochemical impedance spectroscopicmeasurements of fuel cells containing the various MEAs described in theworking and comparative examples. Measurements were made after the MEAshad been conditioned subsequent to start up and attaining stableperformance, typically about 100-150 hours. V150 (FEP25-hp (wet)) wasactually measured after about 2,350 hours, but it had not degraded inperformance versus the earlier measurements with this MEA.

1. A method for manufacturing a membrane electrode assembly using an ionconducting membrane, comprising the steps of: providing anion-conducting membrane in a pre-swollen state being impregnated with aionomer; drying the pre-swollen ion-conducting membrane at elevatedtemperatures in order to remove residual solvent and to transform theionomer into the form of an insoluble solid; after the drying step,re-swelling the ion-conducting membrane by immersing the ion-conductingmembrane in a non-boiling solvent; coating of the ion conductingmembrane on both sides with an electrode layer to form a sandwich; andhot-pressing the sandwich to form an ion conducting bond between theion-conducting membrane and the electrode layers; wherein the ionconducting membrane is impregnated with an ionically conductingpolymeric phase; wherein a duration of the re-swelling step is 4-5hours.
 2. The method according to claim 1, wherein a catalytic activelayer is disposed between the electrode layer and the ion conductingmembrane on both sides of the ion conducting membrane.
 3. Methodaccording to claim 2, wherein the electrode layer comprises one ofcarbon cloth, carbon paper and a carbon felt.
 4. A method according toclaim 2, wherein the electrode layer is exposed to a polar andhydrogen-bonding solvent.
 5. The method according to claim 1, whereinthe electrode layer comprises one of a carbon cloth, carbon paper and acarbon felt.
 6. The method according to claim 1, wherein thehot-pressing condition are selected from at least one of the followingconditions: a) temperature in the range of 70 to 150° C.; b) pressure inthe range of 2 to 30 MPa; and c) duration time of hot-pressing treatmentin the range of 15 to 400 seconds.
 7. The method according to claim 1,wherein the catalytic active layer comprises at least one selected fromthe group containing platinum, ruthenium, rhodium, rhenium, nickel, rareearth and transition metals and compounds thereof.
 8. A membraneelectrode assembly, manufactured according to claim 1, comprising a hotpressed sandwich comprising: a first electrode layer; a second electrodelayer; and an ion conducting membrane disposed between the first andsecond electrode layers; wherein the ion conducting membrane is in apre-swollen status prior to the hot-pressing.
 9. The membrane electrodeassembly according to claim 8, wherein the depth of the ion conductingmembrane is in the range of 5 to 250 μm.
 10. A membrane electrodeassembly according to claim 8, wherein a depth of the ion conductingmembrane is in the range of 20 to 200 μm.
 11. A method according toclaim 1, wherein the ion conducting membrane is exposed to a polar andhydrogen-bonding solvent.
 12. A method according to claim 1, wherein thehot-pressing conditions are selected from at least one of the followingconditions: a) temperature in the range of 90 to 120° C.; b) pressure inthe range of 5 to 18 MPa; and c) duration time of the hot-pressingtreatment in the range of 60 to 240 seconds.
 13. The method of claim 1,wherein the re-swelling is performed in water at approximately 80° C.14. A method for manufacturing a membrane electrode assembly using anion-conducting membrane, comprising steps of: swelling theion-conducting membrane by immersing the ion-conducting membrane in anionomer solution; after the swelling step, drying the ion-conductingmembrane at elevated temperatures in a range from 120 to 140° C. so asto transform the ionomer into an insoluble solid, and so that the ionconducting membrane is impregnated with an ionically conductingpolymeric phase; after the drying step, re-swelling the ion-conductingmembrane by immersing the ion-conducting membrane in a non-boilingsolvent; coating the ion-conducting membrane on both sides with anelectrode layer to form a sandwich; and hot-pressing the sandwich toform an ion conducting bond between the ion-conducting membrane and theelectrode layers; wherein the re-swelling step is performed using waterat room temperature.
 15. The method of claim 14, wherein thehot-pressing step is performed while the ion-conducting membrane isstill in a wet state from the re-swelling step.